Channeled biomedical foams and method for producing same

ABSTRACT

The present invention provides a biomedical, biocompatible, polymeric foam scaffold suitable for use in the repair and regeneration of tissue and which contains located therein a network of, branched channels that are effective to encourage and facilitate vascularization and tissue growth within the scaffold and to methods for making such biomedical scaffolds.

FIELD OF THE INVENTION

[0001] The present invention relates to biomedical porous polymeric foamscaffolds useful for tissue repair and regeneration and methods forpreparing same.

BACKGROUND OF THE INVENTION

[0002] Over the past two decades, the field of tissue engineering hasfocused on the repair and reconstruction of tissue utilizing scaffolds,both as a means to culture cells in vitro for subsequent implantation invivo and as an acellular implant to encourage tissue ingrowth andincorporation. Scaffolds seeded and cultured with cells are utilized todeliver and/or direct cells to desired sites in the body, to define apotential space for engineered tissue, and to guide the process oftissue development. In the case of cell culture, cell transplantation,on or from scaffolds, has been explored for the regeneration of skin,nerve, liver, pancreas, cartilage, adipose and bone tissue, usingvarious biological and synthetic materials.

[0003] Acellular scaffolds have also been developed for promoting theattachment and migration of cells from the surrounding living tissue tothe surface and interior of the scaffold. In these cases, bioabsorbablematerials are useful in order to provide a substrate for incipienttissue growth and subsequent degradation and elimination from the arealeaving behind newly regenerated tissue. Examples of such materialsinclude-poly(lactic acid) (PLA), poly(caprolactone) (PCL), poly(glycolicacid) (PGA), poly(dioxanone) (PDO), poly(trimethylene carbonate) (TMC),and their copolymers and blends.

[0004] Scaffolds, whether acellular or seeded, have certain requirementswith regards to the penetration of the scaffold by cells and thenutrient flow to cells. Scaffolds with pores of diameters up to 500microns provide sufficient open space for the formation of functionaltissue, but lack the means necessary to provide sufficient infiltrationof cells, diffusion of nutrients and oxygen to the cells, removal ofmetabolic waste away from the cells, and to guide the cells and fluids.

[0005] Several attempts to provide scaffolds with architectures toimprove the diffusion of nutrients through the scaffold have been madein the recent past. These include bimodal porous structures that enhancethe available surface area and internal volume of the scaffold. Thesestructures were created using leachable particles incorporated intoeither a polymer or a polymer solution. In the case of the polymersolution, freeze drying was used to create a polymer foam embedded withleachable particles. The foam was then subjected to a subsequent step inwhich the particles were leached out of the system to create a secondset of pores.

[0006] Alternatively, biocompatible porous polymer membranes wereprepared by dispersing salt particles in a biocompatible polymersolution. The solvent was evaporated and the salt particles were leachedout of the membrane by immersing the membrane in a solvent for the saltparticles. A three-dimensional porous structure was then manufactured bylaminating the membranes together to form the desired shape.

[0007] Others have circumvented the use of leachable particles to formporous membranes of various pore diameters by casting a layer of polymersolution on a substrate and submerging the layer/substrate in anon-solvent for the polymer. This created a porous polymer structure.The cast layers were laminated to achieve gradients in porosity in thethree-dimensional structure.

[0008] Still others have used a rigid-coil/flexible-coil block copolymermixed with a solvent that selectively solubilized one of the blocks. Theother block of the copolymer was permitted to self-assemble intoorganized mesostructures. The solvent was then evaporated, leaving thestructure mesoporous.

[0009] The field of tissue engineering to repair and reconstruct tissuehas utilized scaffolds to encourage tissue ingrowth and incorporation,scaffolds in the form of porous polymer foams. The morphology of foamshas progressed from random to controlled formation, but the controlledmorphology has resulted either in a monomodal, isotropic distribution ofpores through spinodal decomposition of polymer solvent mixtures or inthe production of uniaxial channels in the foam. There remains a needfor biodegradable porous polymer scaffolds for tissue engineering thathave an architecture providing for the effective and thoroughdistribution of fluids and nutrients necessary for tissue growth. Inaddition, it would be advantageous to be able to produce this scaffoldby way of a method that does not require any manipulation of thematerial post-processing.

SUMMARY OF THE INVENTION

[0010] The present invention provides a biomedical, biocompatible, foamscaffold suitable for use in the repair and regeneration of tissue thatcomprises a network of branched, channels effective to encourage andfacilitate vascularization and tissue growth therein and a process formaking the biomedical scaffolds. The process comprises preparing ahomogenous mixture of a synthetic, biocompatible polymer, a solvent inwhich the polymer is soluble and a non-solvent in which the polymer isnot soluble. The solvent and non-solvent are miscible and the freezingpoint of the non-solvent is higher than the freezing point of thesolvent. The homogeneous mixture is placed in a mold and cooled to atemperature effective to freeze the non-solvent. This temperature ismaintained for a time effective to allow the non-solvent tophase-separate from the mixture. The mixture is then cooled to atemperature effective to form a solid, and the solvent and non-solventare removed from the solid to provide a biocompatible, porous scaffoldsuitable for use in the repair and regeneration of tissue comprising anetwork of branched channels. This network of channels provide a highdegree of interconnectivity that aids in transferring nutrients to thecenter of the scaffold, thus encouraging and facilitatingvascularization and, ultimately, tissue growth within the scaffoldstructure.

BRIEF DESCRIPTION OF THE FIGURES

[0011]FIG. 1 is a sectional view of a foam scaffold according to thepresent invention.

[0012]FIG. 2 is a scanning electron micrograph of a cross-section of afoam scaffold according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0013] The present invention provides a biocompatible foam scaffold foruse in the repair and regeneration of tissue and a method of producing ascaffold in which a network of branched channels is embedded. Theprocess involves a combination of phase separation and lyophilization inorder to achieve this novel internal architecture.

[0014] According to one aspect of the present invention, a biocompatibleporous scaffold is provided having a substantially continuous polymericfoam phase with a highly interconnected distribution of pores betweenabout 50 and about 500 microns in diameter, in which is embedded anetwork of branched channels. The presence of the branched, channelednetwork in the porous scaffold provides passageways between the poresfor the distribution of nutrients and the removal of waste. Theresulting foams have a porosity of about 90%.

[0015] The branched channels provide interconnectivity that is usefulfor transmitting cell-to-cell, signaling molecules across the scaffoldand allowing for the diffusion of nutrients through the scaffold. Thechanneled network also provides a patterned surface that is useful forguiding cell growth. In addition, the large surface area in the overallfoam is ideal for cell seeding, cell growth and the production ofextracellular matrices. Finally, the existence of channels in athree-dimensional structure encourages cell growth in the pores andfurther provides a means of infiltration into the interior of thescaffold by way of the channels.

[0016] In another aspect of the present invention, the polymer phase isbioabsorbable. Here, the scaffold undergoes biodegradation as the tissuegrows and becomes incorporated in the site of the implantation.

[0017] Referring to FIGS. 1 and 2, scaffold 10 includes a polymeric foamcomponent 12 including pores 14 with open cell pore structure.Continuous branched channels 16 are embedded in foam component 12. Thesebranched channels 16 have primary branches 18 as well as secondarybranches 20. The branched channels 16 in the three-dimensional scaffold10 structure encourages and facilitates cell growth in pores 14 andfurther provides a means for transferring nutrients to the center ofscaffold 10, thus encouraging and facilitating vascularization intoscaffold 10.

[0018] Biomedical polymers are suitable for use in the presentinvention. These types of polymers are biocompatible at the time ofimplant, causing no harm to living tissue. Preferably, the polymersshould be biodegradable, where the polymer degradation products arebiocompatible, non-toxic and physiologically compatible, and may also bebioabsorbable, or resorbed into living tissue. Additional parametersthat play an important role include the mechanical properties of thematerial, especially its mechanical rigidity. High rigidity isadvantageous where cells growing within the scaffold exert forces. It isalso important that the biodegradation kinetics of the polymer match therate of the healing process. Finally, from a processing standpoint, thethermal properties of the polymer are important to allow the polymer toretain mechanical integrity post-processing, e.g. a sufficiently highglass transition temperature to avoid pore/channel collapse upon solventremoval.

[0019] Polymers that can be used for the preparation of scaffolds foruse in the repair and regeneration of tissue according to the presentinvention include polymers selected from the group consisting ofaliphatic polyesters, poly(amino acids), polyalkylenes oxalates,polyamides, tyrosine derived polycarbonates, polyorthoesters,polyoxaesters, poly(anhydrides), and blends thereof. For the purpose ofthis invention aliphatic polyesters include, but are not limited to,homopolymers and copolymers of lactide (which includes lactic acid,D-,L- and meso lactide), glycolide (including glycolic acid),ε-caprolactone, p-dioxanone (1,4-dioxan-2-one), trimethylene carbonate(1,3-dioxan-2-one), alkyl derivatives of trimethylene carbonate,δ-valerolactone, β-butyrolactone, γ-butyrolactone, ε-decalactone,hydroxybutyrate (repeating units), hydroxyvalerate (repeating units),1,4-dioxepan-2-one (including its dimer1,5,8,12-tetraoxacyclotetradecane-7,14-dione), 1,5-dioxepan-2-one, and6,6-dimethyl-1,4-dioxan-2-one and blends thereof.

[0020] Elastomeric copolymers are also particularly useful in thepresent invention. These elastomeric copolymers will have an inherentviscosity in the range of about 1.0 dL/g to 4 dL/g, more preferablyabout 1.0 dL/g to 2.0 dL/g and most preferably about 1.0 dL/g to 1.7dL/g as determined at 25° C. in a 0.1 gram per deciliter (g/dL) solutionof polymer in hexafluoroisopropanol (HFIP). For the purpose of thisinvention, an “elastomeric copolymer” is defined as a polymer which, atroom temperature, can be stretched repeatedly to at least about twiceits original length and which, upon immediate release of stress, willreturn to approximately its original length.

[0021] Exemplary bioabsorbable, biocompatible elastomers include, butare not limited to, elastomeric copolymers of lactide (includingL-lactide, D-lactide, blends thereof, and lactic acid polymers andcopolymers) and ε-caprolactone where the mole ratio of lactide toε-caprolactone is from about 30/70 to about 65/35, and more preferablyfrom about 30/70 to about 50/50; elastomeric copolymers of lactide(including L-lactide, D-lactide, blends thereof, and lactic acidpolymers and copolymers) and glycolide (including polyglycolic acid)where the mole ratio of lactide to glycolide is from about 75/25 toabout 95/5.

[0022] The scaffolds of the present invention are foams prepared by thelyophilization of a polymer dissolved in a homogeneous mixture ofcomponents that act as solvents and non-solvents for the polymer. Theprocess according to the present invention employs thermally inducedphase separation to fabricate highly porous foam scaffolds comprising anetwork of branched channels embedded therein for the optimization ofproperties necessary to encourage and facilitate vascularization. Phaseseparation will occur by liquid-liquid demixing and crystallizationbased on the thermodynamics of the system. After lyophilization, theresulting foam contains an embedded network of branched channelsresulting from the phase separation and crystallization of thenon-solvent from the mixture during processing.

[0023] As mentioned above, the homogeneous mixture is comprised ofseveral components, each of which has a particular relationship with thepolymer. Specifically, the mixture comprises at least a first componentin which the polymer is soluble, referred heretofore as the solvent, anda second component in which the polymer is not soluble, referredheretofore as the non-solvent.

[0024] Typically, the two components are liquids at, or slightly above,room temperature, and must also conform to certain criteria relative toone another. First, the solvent and non-solvent must be miscible; i.e.the solvent solubilizes both the polymer and the non-solvent. Second,the melting point of the non-solvent must be above that of the solvent.The higher melting point of the non-solvent allows phase separation ofthe non-solvent from the polymer mixture upon cooling. This is necessaryto create the channels in the polymer foam. Finally, the freezing pointsof the solvent and non-solvent must be sufficiently disparate in orderthat the solid phase of the non-solvent is favorable, while the solidphase of the solvent is unfavorable, so that complete phase separationmay occur in a given temperature range between the two freezing points.In this temperature range, phase separation occurs as the non-solventcrystallizes from the mixture. Due to colligative properties of themixture, the solvent and non-solvent are expected to undergo a freezingpoint depression, the magnitudes of which are dependent on theproperties of the pure solvent and non-solvent. As a result of thisfreezing point depression, a spread in the freezing points is alsouseful to allow the complete phase separation and crystallization of thenon-solvent from the homogeneous mixture.

[0025] In processes for making the scaffolds of the present invention, ahomogeneous mixture is made by combining polymer, solvent, andnon-solvent. The mixture is then poured into a mold and placed in alyophilizer. The formation of the channeled scaffold is a one-stepprocess that relies on the properties of the respective components ofthe mixture and the lyophilization cycle. The first segment of thecooling cycle involves a ramping-down to a temperature below thedepressed freezing point of the non-solvent, but above that of thesolvent. The temperature is held while crystals of the non-solvent beginto nucleate and grow. This phase separation may occur in the form ofdendrites of the non-solvent that grow in the mixture, creating abranch-like structure within the mixture. Since the polymer is notsoluble in the non-solvent, the dendrites of non-solvent do not containpolymer. The dendrites of non-solvent growing in the polymer solutionact as placeholders for the branched channel network structure that willresult upon the sublimation of the non-solvent.

[0026] Once the non-solvent has crystallized, the temperature isdecreased below the freezing point of the solvent. At this point, thepolymer/solvent mixture solidifies. The degree of crystallinity of thissolid God phase depends on the rate of temperature decrease. Whetheramorphous, crystalline, or some combination thereof, this solidifiedpolymer/solvent mixture is responsible for foam formation.

[0027] The solid mixture sublimes by way of the lyophilization cycle asvacuum is applied to the frozen sample, leaving behind a polymer foamwith pores forming as the solvent sublimes and branched channels formingin the foam as the non-solvent sublimes.

[0028] Non-solvent crystallization during liquid-liquid demixing occurswith the correct selection of solvents and processing conditions. Amixture of at least one component which is a solvent and at least onecomponent which is a non-solvent for the polymer is employed. Asmentioned above, the solvent and non-solvent must be miscible. Further,the proportions and mixing of the two components are chosen so as toretain solubility of the polymer in the solvent despite its insolubilityin the non-solvent. This yields a uniform, homogenous mixture.

[0029] Solvents useful in the present invention include, but are notlimited to, dimethyl carbonate (DMC; m.p. 5° C.), 1,4-dioxane (m.p. 12°C.) and diethyl carbonate (m.p. −43° C.). Non-solvents in which thepolymer is insoluble that are suitable for use include, but not limitedto, alcohols such as t-butanol (m.p. 25° C.), tert-amyl alcohol (m.p.−12° C.), 3,3 dimethyl-2 butanol (m.p. −5° C.), octanol (m.p. −15° C.),nonanol (m.p. −8° C.), decanol (m.p. 7° C.), n-decanol (m.p. 11° C.),and dodecanol (m.p. 22-26° C.). It is critical that the polymer besoluble in the overall solvent/non-solvent mixture.

[0030] The melting point of the non-solvent must be higher than that ofthe solvent. In addition, the disparity in melting points between thetwo solvents should be sufficiently large to allow thoroughcrystallization of the non-solvent during liquid/liquid demixing step ofthe process. Preferably, the disparity in melting points is greater thanabout 20° C. A preferred solvent/non-solvent pair meeting the aboverequirements are dimethyl carbonate (DMC; m.p. 5° C.) which is a solventfor the disclosed polymers, and t-butanol (m.p. 25° C.), a non-solventfor the disclosed polymers.

[0031] Combining the polymer, solvent and non-solvent can beaccomplished in two ways. The polymer may first be completely dissolvedin the solvent followed by the addition of the non-solvent.Alternatively, the solvent and non-solvent may be mixed followed by theaddition of polymer. If the non-solvent is added to the polymersolution, then it is added into a constantly agitated polymer solutionat a rate effective to avoid localized precipitation of the polymer inthe homogeneous mixture. The weight ratio of the non-solvent to thetotal volume of the solvent and non-solvent is preferably between about1 to about 50 weight percent and more preferable between about 15 toabout 30 weight percent. The polymer concentration in the solventmixture is preferably between about 0.5 and about 25 weight percent andmore preferably between about 2.5 and about 10 weight percent.

[0032] Alternatively, the method of the present invention can also beused to make a porous scaffold of a first polymer with branched channelsof a second porous polymer embedded within the structure. This structureis created if the second polymer is insoluble in the solvent, andsoluble in the non-solvent. This mixture creates dual solid-liquid phasetransformation processes occurring within the same system. Again, thetwo components must be miscible, but the two polymers used in the systemmust only be soluble in one of the components, i.e. its respectivesolvent. Upon cooling the system, the temperature is held for asufficient time to allow the first component to crystallize, thusforcing a solid-liquid phase transformation and precipitation of thefirst polymer in a dendritic branched fashion. After completecrystallization of the first component, the temperature is then lowereduntil the second solid-liquid phase transformation occurs. Sublimationof the system leaves behind a branched foam structure of one polymerembedded within a second foam structure of another polymer.

[0033] The porous polymer scaffolds can be molded or cut to shape fortissue engineering and tissue guided regeneration applications. Cellularpre-seeding can be used prior to implantation or the scaffold can beused in an acellular fashion due to the structure of the scaffold thatallows generous cellular ingrowth. The scaffold serves both as aphysical support and an adhesive substrate for isolated cells during invitro culture and subsequent implantation. As the transplanted cellpopulations grow, the cells function normally and begin to secrete theirown extracellular matrices (ECM) which allows the scaffold to mimic theECM of an organ. The porous polymer scaffold may, therefore, be used asan external scaffolding for the support of in vitro culturing of cellsfor the creation of external support organs. In all cases, the scaffoldpolymer is selected to degrade as the need for the artificial supportdiminishes.

[0034] In applications where the tissue shape is integral to tissuefunction, the polymer scaffold may be molded to have the appropriatedimensions. Any crevices, apertures or refinements desired in thethree-dimensional structure can be created by fashioning the matrix withscissors, a scalpel, a laser beam or any other cutting instrument.Scaffold applications include the regeneration of tissues such asadipose, pancreatic, cartilaginous, osseous, musculoskeletal, nervous,tendenous, hepatic, ocular, integumeary, arteriovenous, urinary or anyother tissue forming solid or hollow organs.

[0035] The scaffold may also be used in transplantation as a matrix fordissociated cell types. These include fibrochondrocytes, adipocytes,pancreatic Islet cells, osteocytes, osteoblasts, myeloid cells,chondrocytes, hepatocytes, exocrine cells, cells of intestinal origin,bile duct cells, parathyroid cells, nucleus pulposus cells, annulusfibrosis cells, thyroid cells, endothelial cells, smooth muscle cells,fibroblasts, meniscal cells, sertolli cells, cells of theadrenal-hypothalamic-pituitary axis, cardiac muscle cells, kidneyepithelial cells, kidney tubular cells, kidney basement membrane cells,nerve cells, blood vessel cells, cells forming bone and cartilage,smooth muscle cells, skeletal muscle cells, ocular cells, integumentarycells, keratinocytes, peripheral blood progenitor cells, fat-derivedprogenitor cells, glial cells, macrophages, mesenchymal stem cells,embryonic stem cells, stem cells isolated from adult tissue, geneticallyengineered cells, and combinations thereof. Pieces of tissue can also beused, which may provide a number of different cell types in the samestructure.

[0036] Allogeneic or autologous cells may be used and are dissociatedusing standard techniques and seeded onto or into the foam scaffold. Ifthe cells are seeded onto the scaffold, seeding may take place prior to,or after, the scaffold is implanted. If the cells are added afterimplantation, the added benefit is that cells are placed into thescaffold after it has had an opportunity to vascularize and beincorporated into the implant site. Methods and reagents for culturingcells in vitro and implantation of a tissue scaffold are known to thoseskilled in the art.

[0037] After fabrication, scaffolds can be further modified to increaseeffectiveness of the implant. For example, the scaffolds can be coatedwith bioactive substances that function as receptors or chemoattractorsfor a desired population of cells. The coating can be applied throughabsorption or chemical bonding and may be designed to delivertherapeutic or medicated additives in a controlled fashion. In addition,since the lyophilization of the foam takes place at low temperatures,thermally sensitive additives can be used without concern of degradationduring polymer processing. The additive may be released by a bioerosionof the polymer phase or by diffusion from the polymer phase. Alternativeto release, the additive may simply migrate to the polymer surface ofthe scaffold structure where it is active.

[0038] Depending on the additive and the nature of the components usedin the system, the additive may be added to the pre-blended mixture orit may be added first to the component in which it is most solublebefore adding another component. The additive may be provided in aphysiologically acceptable carrier, excipient, or stabilizer, and may beprovided in sustained release or timed release formulations. Theadditives may also incorporate biological agents to facilitate theirdelivery, such as antibodies, antibody fragments, growth factors,hormones, demineralized bone matrix, or other targeting moieties, towhich the additives are coupled.

[0039] Acceptable pharmaceutical carriers for therapeutic use are wellknown in the pharmaceutical field. Such materials are non-toxic torecipients at the dosages and concentrations employed, and includediluents, solubilizers, lubricants, suspending agents, encapsulatingmaterials, solvents, thickeners and dispersants. Also acceptable arebuffers such as phosphate, citrate, acetate and other organic acidsalts. Anti-oxidants such as ascorbic acid, preservatives, low molecularweight peptides (less than about 10 residues), such as polyarginine,proteins such as serum albumin, gelatin or immunoglobulins may also beused. The pharmaceutical carriers can also include hydrophilic polymerssuch as poly(vinylpyrrolindinone), amino acids such as glycine, glutamicacid, aspartic acid or arginine, monosaccharides, disaccarides, andother carbohydrates including cellulose or its derivatives, glusocse,mannose or dextrines. Chelating agents such as EDTA, sugar alcohols,such as mannitol or sorbitol, counter-ions such as sodium and/ornon-ionic surfactants such as tween, pluronics or PEG are all acceptablecarriers as well.

[0040] As mentioned, the coating can be applied through absorption orchemical bonding, the latter taking place by covalently binding theadditive to a pendent free carboxylic acid group on the polymer. Forexample, moieties having reactive functional groups or being derivatizedto contain active functional groups may be reacted with polymer pendentfree carboxylic acid groups to form a polymer conjugate. If the additiveis active in the conjugate form, then conjugates that are resistant tohydrolysis are utilized. The opposite is true if the additive isinactive in the conjugate form in which case the conjugate used ishydrolyzable.

[0041] The amount of additive incorporated into the porous polymerscaffold is chosen to provide optimal efficacy to the subject in need oftreatment, typically a mammal. A dose and method of administration willvary from subject to subject and be dependent upon such factors as thetype, sex, weight, and diet of the mammal being treated. Other factorsinclude concurrent medication, the particular compounds employed,overall clinical condition, and other factors that those skilled in theart will recognize. The porous polymer scaffolds can be utilized invitro or in vivo as tissue engineering and tissue guided regenerationscaffold in mammals such as primates, including humans, sheep, horses,cattle, pigs, dogs, cats, rats, and mice. As the polymers used in thisinvention are typically suitable for storage at ambient or refrigeratedtemperatures, the polymer-drug combinations of this invention may beprepared for storage under conditions suitable for the preservation ofdrug activity. Sterility is also an issue for polymer scaffolds to beused in tissue engineering and tissue guided regeneration applicationsand may be accomplished using conventional methods such as treatmentwith gases, heat, or irradiation.

[0042] Additives suitable for use with the present invention includebiologically or pharmaceutically active compounds. Examples ofbiologically active compounds include cell attachment mediators, such aspeptide containing variations of the “RGD” integrin binding sequenceknown to affect cellular attachment, biologically active ligands, andsubstances that enhance or exclude particular varieties of cellular ortissue ingrowth. Examples of such substances include integrin bindingsequence, ligands, bone morphogenic proteins, epidermal growth factor,fibroblast growth factor, platelet-derived growth factor, IGF-I, IGF-II,TGF-β I-III, growth differentiation factor, parathyroid hormone,vascular endothelial growth factor, hyaluronic acid, gylcoprotein,lipoprotein, and the like.

[0043] Examples of pharmaceutically active compounds includeantiinfectives, analgesics, anorexics, antihelmintics, antiarthritics,antiasthmatics, anticonvulsants, antidepressants, antidiuretics,antidiarrheals, antihistamines, antiinflammatory agents, antimigrainepreparations, antinauseants, antineoplastics, antiparkinsonism drugs,antipruritics, antipsychotics, antipyretics, antispasmodics,anticholinergics, sympathomimetics, xanthine derivatives, calciumchannel blockers, beta-blockers, antiarrhythmics, antihypertensives,diuretics, vasodilators, central nervous system stimulants,decongestants, hormones, steroids, hypnotics, immunosuppressives, musclerelaxants, parasympatholytics, psychostimulants, sedatives,tranquilizers, naturally derived or genetically engineered proteins,polysaccharides, glycoproteins, or lipoproteins, oligonucleotides,antibodies, antigens, cholinergics, chemotherapeutics, hemostatics, clotdissolving agents, radioactive agents and cystostatics, and the like.Therapeutically effective dosages may be determined by in vitro or invivo methods. For each particular additive, individual determinationsmay be made to determine the optimal dosage required. The determinationof effective dosage levels to achieve the desired result will be withinthe realm of one skilled in the art. The release rate of the additivesmay also be varied within the routine skill in the art to determineadvantageous profile, depending on the therapeutic conditions to betreated.

[0044] A typical additive dosage might range from about 0.001 mg/kg toabout 1000 mg/kg, preferably from about 0.01 mg/kg to about 100 mg/kg,and more preferably from about 0.10 mg/kg to about 20 mg/kg. Theadditives may be used alone or in combination with other therapeutic ordiagnostic agents.

[0045] The invention will be better understood by reference to thefollowing non-limiting examples.

EXAMPLE 1

[0046] A mixture to be lyophilized was first prepared. The mixture wascomposed of a 60/40 copolymer of PLA/PCL (I.V. of 1.7 dL/g at 25° C. ina 0.1 g/dL solution of HFIP), and dimethyl carbonate, a solvent for60/40 PLA/PCL, in a 95/5 weight ratio. The polymer and solvent wereplaced into a flask that was then placed into a water bath and heated to70° C. The solution was heated and stirred for 5 hours. Afterwards, thesolution was filtered using an extraction thimble (extra coarseporosity, type ASTM 170-220 (EC)) and stored in the flask.

[0047] Twenty milliliters of t-butanol was added to 80 ml of the polymersolution in a dropwise fashion to form an 80/20 volumeric mixture. Thepolymer solution was constantly agitated during the dropwise addition oft-butanol.

[0048] Twenty milliliters of the 80/20 mixture was poured into a 50-mlrecrystallization dish. The dish was placed on the shelf of a pre-cooled(20° C.) laboratory scale lyophilizer (Model Freeze Mobile G from VirtisCompany (Gardiner, N.Y.), and was subjected to the following freeze drysequence: cool at 2.5° C./min to 0° C., hold 40 minutes; cool at 2.5°C./min to −10° C., hold 120 minutes; cool at 2.5° C./min to −50° C.,hold 15 minutes; hold at −48° C. for an additional 60 minutes; turn onthe condenser; turn on vacuum pump once condenser reaches −40° C.; holduntil vacuum in chamber is 150 mT and vacuum in foreline is 100 mT, thenhold an additional 60 minutes; warm at 2.5° C./min to −30° C., hold 60minutes; warm at 2.5° C./min to −15° C., hold 60 minutes; warm at 2.5°C./min to 0° C., hold 60 minutes; warm at 2.5° C./min to 15° C., hold 60minutes; warm at 2.5° C./min to 22° C., hold 60 minutes.

[0049] As the temperature decreased to −10° C., dendritic crystals grewin the solution as the t-butanol phase separated from the mixture. Theremaining polymer in dimethyl carbonate was frozen at −50° C. The foamwas formed as the dimethyl carbonate sublimed and the channels wereformed as the dendritic crystals of t-butanol sublimed.

[0050] Scanning electron micrographs (SEMs) showed the average porediameter of the foam to be in the range of 50 to 400 microns and thechannels to have an average diameter in the range of 0.5 to 1.0 mm.

We claim:
 1. A biomedical, biocompatible scaffold suitable for use inthe repair and regeneration of tissue, comprising a polymeric foam, saidfoam comprising a network of branched channels effective to encourageand facilitate vascularization and tissue growth in said scaffold. 2.The biomedical scaffold of claim 1 wherein said scaffold isbioabsorbable.
 3. The biomedical scaffold of claim 1 wherein saidscaffold comprises a bioabsorbable polymer selected from the groupconsisting of aliphatic polyesters, poly(amino acids), polyalkylenesoxalates, polyamides, tyrosine derived polycarbonates, polyorthoesters,polyoxaesters, and poly(anhydrides).
 4. The biomedical scaffold of claim3 wherein said polymer comprises an aliphatic polyester selected fromthe group consisting of homopolymers and copolymers of lactide, lacticacid, glycolide, glycolic acid, ε-caprolactone, p-dioxanone,trimethylene carbonate, alkyl derivatives of trimethylene carbonate,δ-valerolactone, β-butyrolactone, γ-butyrolactone, ε-decalactone,hydroxybutyrate, hydroxyvalerate, 1,4-dioxepan-2-one,1,5,8,12-tetraoxacyclotetradecane-7,14-dione, 1,5-dioxepan-2-one, and6,6-dimethyl-1,4-dioxan-2-one.
 5. The biomedical scaffold of claim 4wherein said aliphatic polyester comprises an elastomer selected fromthe group consisting of copolymers of lactide and ε-caprolactone, andlactide and glycolide.
 6. The biomedical scaffold of claim 5 whereinsaid elastomer has an inherent viscosity in the range of about 1 to 2deciliters per gram as determined at 25° C. in a 0.1 gram per decilitersolution of polymer in hexafluoroisopropanol.
 7. The biomedical scaffoldof claim 6 wherein said copolymer of lactide and ε-caprolactonecomprises a mole ratio of lactide to ε-caprolactone from about 30/70 toabout 50/50.
 8. The biomedical scaffold of claim 6 wherein saidcopolymer of lactide and glycolide comprises a mole ratio of lactide toglycolide from about 75/25 to about 95/5.
 9. The biomedical scaffold ofclaim 1 comprising a continuous polymer phase comprising interconnectedpores of between about 50 and about 500 microns in diameter.
 10. Thebiomedical scaffold of claim 1 further comprising cells.
 11. Thebiomedical scaffold of claim 1 further comprising a bioactive substance.12. A process for making biomedical, biocompatible scaffolds suitablefor use in the repair and regeneration of tissue, comprising: preparinga homogenous mixture comprising a synthetic, biocompatible polymer, asolvent in which said polymer is soluble, and a non-solvent in whichsaid polymer is not soluble, wherein said solvent and said non-solventare miscible, and wherein the freezing point of said non-solvent ishigher than the freezing point of said solvent, placing said homogenousmixture in a mold or other device suitable for preparing foam scaffoldssuit-able for use in repair and regeneration of tissue, cooling saidhomogenous mixture to a first temperature effective to freeze saidnon-solvent, maintaining said first temperature for a time effective toallow phase separation of said non-solvent from said homogenous mixture,cooling said homogenous mixture to a second temperature sufficient toform a solid, and removing said solvent and said non-solvent from saidsolid to provide a biocompatible, porous foam scaffold which comprises anetwork of branched channels.
 13. The process of claim 12 wherein saidpolymer is bioabsorbable.
 14. The process of claim 13 wherein saidbioabsorbable polymer is selected from the group consisting of aliphaticpolyesters, poly(amino acids), polyalkylenes oxalates, polyamides,tyrosine derived polycarbonates, polyorthoesters, polyoxaesters andpoly(anhydrides).
 15. The process of claim 14 wherein said polymercomprises an aliphatic polyester selected from the group consisting ofhomopolymers and copolymers of lactide, lactic acid, glycolide, glycolicacid, ε-caprolactone, p-dioxanone, trimethylene carbonate, alkylderivatives of trimethylene carbonate, δ-valerolactone, β-butyrolactone,δ-butyrolactone, ε-decalactone, hydroxybutyrate, hydroxyvalerate,1,4-dioxepan-2-one, 1,5,8,12-tetraoxacyclotetradecane-7,14-dione,1,5-dioxepan-2-one, and 6,6-dimethyl-1,4-dioxan-2-one.
 16. The processof claim 15 wherein said aliphatic polyester comprises an elastomerselected from the group consisting of copolymers of lactide andε-caprolactone, lactide and glycolide, and blends thereof.
 17. Theprocess of claim 16 wherein said copolymer of lactide and ε-caprolactonecomprises a mole ratio of lactide to ε-caprolactone from about 30/70 toabout 50/50.
 18. The process of claim 16 wherein said copolymer oflactide and glycolide comprises a mole ratio of lactide to glycolidefrom about 75/25 to about 95/5.
 19. The process of claim 14 wherein saidsolvent is selected from the group consisting of dimethyl carbonate,1,4-dioxane and diethyl carbonate.
 20. The process of claim 19 whereinsaid non-solvent is selected from the group consisting of t-butanol,tert-amyl alcohol, 3,3 dimethyl-2 butanol, octanol, nonanol, decanol,n-decanol and dodecanol.